From Wikipedia, the free encyclopedia

A mole (also called a defector in
place, an informant and in the Mafia a
rat) is a spy who works for an enemy nation, but whose loyalty
ostensibly lies with his own nation's government. In some usage, a mole differs
from a defector in that a mole is a spy before
gaining access to classified information, while a defector becomes
a spy only after gaining access. However, others use the term mole
to describe any agent of a foreign power within a government
organization.

Perhaps the most famous examples of moles are the Cambridge Five,
five men recruited as students at Cambridge University who later
rose to high levels in various parts of the British government.
Because of the long preparation time and the difficulty of
inserting moles, they are quite rare in the top levels of
espionage. For instance, there is evidence of only one mole ever
penetrating the CIA: Karl Koecher. By contrast, Aldrich Ames became
an intelligence officer and then Director of Counterintelligence
before he offered his services to the KGB.

Moles are far more common in police work, where they are known as undercover officers. The mole in spying is
mostly used as a last resort due to its value.

The term "mole" is also commonly used to describe anyone working
in one organization, seeking access to confidential information
that they will pass to the organization for whom they really work.
For example, a news reporter seeking information on a company's
employment practices (such as its use of illegal aliens) may
obtain a job with the company to observe the practices
first-hand.